Pressure sensitive adhesives (PSAs) are an important class of materials. Generally, PSAs adhere to a substrate with light pressure (e.g., finger pressure) and typically do not require any post-curing (e.g., heat or radiation) to achieve their maximum bond strength. A wide variety of PSA chemistries are available including, e.g., acrylic, rubber, and silicone based systems. Silicone PSAs offer one or more of the following useful characteristics: adhesion to low surface energy (LSE) surfaces, quick adhesion with short dwell times, wide use temperature (i.e., performance at high and low temperature extremes), weathering resistance (including resistance to ultraviolet (UV) radiation, oxidation, and humidity), reduced sensitivity to stress variations (e.g., mode, frequency and angle of applied stresses), and resistance to chemicals (e.g., solvents and plasticizers) and biological substances (e.g., mold and fungi).
Generally, silicone pressure sensitive adhesives have been formed by a condensation reaction between a polymer or gum and a tackifying resin. The polymer or gum is typically a high molecular weight silanol-terminated poly(diorganosiloxane) material e.g., silanol-terminated poly(dimethylsiloxane) (“PDMS”) or poly(dimethylmethylphenylsiloxane). The tackifying resin is typically a three-dimensional silicate structure end-capped with trimethylsiloxy groups. In addition to the terminal silanol groups of the polymer or gum, the tackifying resin may also include residual silanol functionality.
Such systems rely on high molecular weight starting materials; thus, they must be diluted in solvents to achieve viscosities suitable for coating at room temperature. Typical coatable solutions contain less than 60% solids by weight in a solvent (e.g., an aromatic solvent such as toluene or xylene). Additional solvent may be added prior to coating such that volatile organic compound (VOC) contents of greater than 50% are common when using traditional silicone PSAs.
A number of approaches have been investigated for the low VOC delivery of silicone PSAs. For example, water-based emulsion systems and liquid solventless systems based on low-viscosity, highly-functional silicone polymers have been explored, e.g., polymers including silicon-bonded hydrogen, silicon-bonded vinyl, silicon-bonded epoxy, and silicon-bonded acrylate. Hot-melt, moisture-curable, silicone PSAs that rely on silicon-bonded hydrolysable functional groups (e.g., alkoxy, acetoxy, or oxime groups) have also been attempted.
Despite these advances, there is still a need for more robust methods for the low VOC delivery of silicone PSAs. There is also a need for low VOC delivery processes that allow for a greater diversity of silicone chemistries to be used, thus enabling a broader range of end-use performance properties.
While some silicone PSA formulations provide acceptable performance after solvent removal, some systems benefit from additional crosslinking. Conventional silicone PSAs are cured by thermal processes using specific types of catalysts. For example, platinum catalysts have been used with addition cure systems, peroxides (e.g., benzoyl peroxide) have been used with hydrogen-abstraction cure systems, and tin catalysts have been used with moisture/condensation cure systems.
Generally, some of these approaches require reactive functional groups attached to the siloxane backbone. For example, addition-cure, platinum-catalyzed systems generally rely on a hydrosilation reaction between silicon-bonded vinyl functional groups and silicon-bonded hydrogens. In general, it may be desirable to have a silicone adhesive system that can be cured without the use of catalysts, particularly when hot-melt coating or in other circumstances where premature curing should be avoided.